1. Field of the Invention
The present invention relates in general to a semiconductor device, such as an MOS transistor, in which there is compensation for the drop in the threshold voltage (Vth) due to the short-channel effects, and to a process for fabrication of such a semiconductor device.
2. Description of the Related Art
For a given nominal channel length (L) of a transistor, the threshold voltage (Vth) drops suddenly, in particular for short-channel transistors (i.e., those having a channel length of less than 0.25 xcexcm and typically a channel length, L, of about 0.18 xcexcm).
The threshold voltage of a semiconductor device such as an MOS transistor, in particular a short-channel device, is a critical parameter of the device. This is because the leakage current of the device (for example, of the transistor) depends strongly on the threshold voltage. Taking into consideration current supply voltages and those envisaged in the future (from 0.9 to 1.8 volts) for such devices and the permitted leakage currents (Ioff of approximately 1 nA/xcexcm), the threshold voltage Vth must have values of approximately 0.2 to 0.25 volts.
The sudden voltage drop (or roll-off) in the zones of the channel region of the semiconductor device results in dispersion of the electrical characteristics of the device and makes it difficult to obtain the desired threshold voltages.
To remedy this threshold voltage roll-off in semiconductor devices such as MOS transistors, it has been proposed, as described in the article xe2x80x9cSelf-Aligned Control of Threshold Voltages in Sub-0.02-xcexcm MOSFETsxe2x80x9d by Hajima Kurata and Toshihiro Sugii, IEEE Transactions on Electron Devices, Vol. 45, No. Oct. 10, 1998, to form, in the channel region, pockets adjacent to the source and drain region junctions that have a conductivity of the same type as the substrate; but in which, the dopant concentration is greater than that of the substrate.
Although this solution reduces the threshold voltage roll-off gradient in the channel region, the short-channel effects lead to a more rapid roll-off of the threshold voltage, Vth, than the increase in the threshold voltage that can be obtained by incorporating the compensation pockets of the prior art.
Consequently, although these compensation pockets allow partial local compensation for the roll-off of the threshold voltage, Vth, it is not possible to obtain complete compensation for the roll-off over the entire channel region range desired.
Therefore a semiconductor device, such as an MOS transistor, that remedies the drawbacks of the devices of the prior art may be desired.
More particularly, a semiconductor device, such as an MOS transistor, whose voltage threshold roll-off due to the short-channel effects is almost fully compensated for may be desired. This makes it possible to achieve channel lengths which are arbitrarily small but non-zero.
Also a semiconductor device, such as an MOS transistor, may have a constant threshold voltage, Vth, when the channel length, L, decreases down to very small effective channel lengths, for example, 0.025 xcexcm or less.
A process for fabricating a semiconductor device may apply to devices having channels of arbitrarily small length, these being, moreover, technologically realizable.
A semiconductor device is described that may have a semiconductor substrate with a predetermined concentration, Ns, of a dopant of a first conductivity type. The device may have source and drain regions which are doped with a dopant of a second conductivity type, which is opposite of the first conductivity type. Junctions delimiting a channel region of predetermined nominal length, LN, may be defined in the substrate. A first pocket adjacent to each of the junctions and having a predetermined length, Lp, may be defined. The first pockets may be doped with a dopant of the first conductivity type but with a local concentration, Np, which locally increases the net concentration in the substrate. The device may include at least one second pocket located adjacent to each of the junctions and stacked against each of the first pockets. These second pockets may have a length, Ln, such that Ln greater than Lp. The second pockets may be doped with a dopant of the second conductivity type and have a concentration, Nn, such that Nn less than Np. This may locally decrease the net concentration of the substrate without changing the conductivity type.
In an embodiment, the second pockets include a plurality of elementary pockets stacked against one another. Each elementary pocket of a given rank, i, may have a predetermined length, Lni, and a predetermined concentration, Nni, of a dopant of the second conductivity type satisfying the following relationships:
Ln1 greater than Lp,
Lnixe2x88x921 less than Lni less than Lni+1,
Nnixe2x88x921 greater than Nni greater than Nni+1, and
the sum, xcexa3Nni, of the concentrations of the dopant of the second conductivity type in the elementary pockets may be such that:
xcexa3Nni less than Ns.
In other words, the second pockets decrease the net concentration of dopant of the first conductivity type both in the first pockets and in the channel region. However, they do not change the conductivity type of the first pockets nor of the channel region.
A process for fabricating a semiconductor device as defined above is described. The process may include the formation of a source region and of a drain region in a semiconductor substrate having a predetermined concentration, Ns, of a dopant of a first conductivity type. The source region and the drain region may be doped with a dopant of a second conductivity type, which is opposite of the first conductivity type. The source and drain regions may form one or more junctions in the substrate such that the junctions delimit between them a channel region. The channel region may have a predetermined nominal length, LN. In the channel region in a zone adjacent to each of the junctions, one or more first pockets may be formed having a predetermined length, Lp, and a predetermined concentration, Np. This may locally increase the net concentration in the substrate above Ns. The process may furthermore include the implantation, in the channel region, of a dopant of the second conductivity type, which is opposite of the first conductivity type. This may be done under a set of conditions such that at least one second pocket is formed in the channel region. Each second pocket may be stacked against each of the first pockets, respectively. The second pocket may have a length, Ln, such that Ln greater than Lp, and a concentration, Nn, of a dopant of the first type such that Nn less than Np. This may locally decrease the net concentration in the substrate, without changing the conductivity type.
In a preferred embodiment, the implantation of the dopant of the second conductivity type consists of a series of successive implantations under a set of conditions such that the second pockets formed each consist of a plurality of elementary pockets stacked against one another. Each elementary pocket of a given rank, i, may have a length, Lni, and a concentration, Nni, of a dopant of the second conductivity type satisfying the relationships:
Ln1 greater than Lp,
Lnixe2x88x921 less than Lni less than Lni+1,
Nnixe2x88x921 greater than Nni greater than Nni+1, and
the sun, xcexa3Nni, of the concentrations of the dopant of the second conductivity type in the elementary pockets being such that:
xcexa3Nni less than Ns.
The lengths Lp and Ln of the pockets are taken from the junctions.
Implantation of a dopant in a semiconductor substrate is a known process and it is possible, in the present process, to use any implantation process conventionally used in the technology of semiconductors.
As is known, the formation of doped pockets in a semiconductor substrate depends on the angle of incidence of the implantation with respect to the normal to the substrate, on the implantation dose, and on the implantation energy of the dopant. Thus, by varying the angle of incidence and the dopant dose, it is possible to increase the length of the implanted pocket and to vary the dopant concentration.
As a variant, in order to vary the length of the second implanted pockets and their dopant concentration, successive implantation steps may be carried out with the same angle of incidence with respect to the normal, the same dose, and the same implantation energy. However, subjecting the device to a different annealing heat treatment step after each successive implantation step may make the dopant implanted in the substrate diffuse differently for each implanted pocket.